chore: clean up builder2, Ceph module, normalize Darwin host symlinks

- Remove builder2 (retired host): dangling network.json entry and
  empty home/hosts/builder2 stub. Its old IP (10.42.1.17) is already
  correctly owned by pve4.
- Remove the abandoned Ceph module (modules/nixos/ceph.nix) and its
  unencrypted plaintext keyring files under secrets/. No host ever
  enabled services.ceph-benaco; the keyrings were dead, unencrypted
  credentials sitting in the repo.
- Normalize Darwin host identity: IVR and Lithic are the only two
  physical Darwin machines. All other darwin/hosts/* names are now
  symlinks to whichever of the two they represent, matching the DHCP
  name variations nix-darwin sees depending on network:
    gregory -> ivr
    gregory.hellings-mbp -> ivr
    MacBook-Prolocal -> ivr
    gregs-MacBook-Pro-16-inch-Nov-2024 -> lithic
    li -> lithic (pre-existing)
  This lets each machine's config be maintained once regardless of
  what hostname it currently advertises.

network.json

@@ -9,10 +9,6 @@
     "tailscale": "100.64.0.0/10"
   },
   "hosts": {
-    "builder2": {
-      "ip": "10.42.1.17",
-      "system": "x86_64-linux"
-    },
     "exodus": {
       "ip": null,
       "pubkey": "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIFxmnCj2E9DxcnefPW+n4yCuLShxqr0p024riogdeXA3",